An important form of radio frequency (RF) power divider is the 3db hybrid coupler which is described in a number of references including Chapter 13 entitled “TEM-Mode, Coupled-Transmission-Line Directional Couplers, and Branch-Line Directional Couplers” of a book whose title is “Microwave Filters, Impedance-Matching Networks And Coupling Structures” by Matthaei, Young and Jones. The 3db hybrid coupler has two input ports and two output ports. With one input connected to a terminating impedance matched to the system characteristic impedance, a signal at the other input produces signals at the two outputs of the coupler each of which contains approximately one-half of the power engendered by the input signal (neglecting insertion loss). Depending on device form and connections, the outputs may differ in phase from each other by 0, 90, or 180 degrees. The 90 degree phase type sometimes is called a quadrature hybrid.
The Magic-T or Rat-Race hybrid ring circuit is another type, which throughout the past has been optimized with the purpose of obtaining a higher bandwidth (>40%). Various approaches to increase the bandwidth include using non-flat technology instead of the middle wave length line (asymmetric part) of the ring. The resulting ring is more symmetrical and the bandwidth is only limited by the interconnection of the quarter length wave sections. The hybrid ring can be described as a divider or 180 degree coupler, and is particularly useful in mixer and coupling signal circuits.
Generally then, the 0 degree hybrid coupler is a four-port network available from a number of manufacturers in a wide variety of package types, ranging over a frequency spectrum of 10 kHz to 18 GHz. The traditional function of a 0 degree hybrid coupler is to split an input signal into two equal amplitude, isolated 0 degree outputs or to combine to similarly phased, equal amplitude signals into a single output.
Operationally, a 0 degree hybrid coupler is a symmetrical network in that signals applied to any port will split equally between the opposite port pairs. An input signal applied to port 1 will split equally between ports 2 and 3. The output signals from ports 2 will be in-phase with the input signal at port 1. The input signal is split equally so that the two resulting output signals. An important natural characteristic of a 0 degree hybrid coupler is its reaction to mismatches. In the case of a common input mismatch, all reflections are directed to the isolated port 4, and as a result system match is not affected when port 4 is terminated in its characteristic impedance. The same condition holds true for output mismatches, reflections are directed to the isolated port 4. The standard hybrid coupler may also be used to combine two signals at ports 2 and 3 into an output signal at port 1.
U.S. Pat. No. 1,458,193, entitled “Multiple Balancing Arrangement For Multiplex Transmission”, to Osbourne, describes a transformer type of the hybrid junction. In this type, transformer windings are tapped to create uncoupled ports, in like fashion to bridge circuits, such as the Wheatstone Bridge. Tapped winding hybrid transformers have found wide application, especially in telephone repeaters. One such 2 way amplifier, or “repeater”, is described by Wright in U.S. Pat. No. 1,515,643, entitled “Transmission Circuits”. This hybrid became key to long distance telephony, and they remain in use today, as for instance to reduce earpiece “sidetone” in telephone handsets to comfortable levels.
Unfortunately however, limitations can arise in tapped winding hybrid transformers. The multiplicity of windings is complex: a magnetic circuit or “core” is needed to ensure coupling between all, and in essence, 6 windings are required. Any number of difficulties can upset symmetry, causing unbalance. For instance, if the taps are not at the midpoint of the winding turns, or the magnetic core has a void, isolation between ports 1 and 4 is reduced. And since winding techniques vary, intricate care may be required in practice to achieve high isolation.
Hybrid junctions of the transformer type generally require a magnetic circuit or ferrite core, which limits frequency response. They may not operate above say 1 Ghz, or are narrowband above this. They also have limited power ratings and complex geometry, such as six windings and many cores.
What is needed then, is a more simple and uncomplicated hybrid junction, one that obtains 4 ports from 4 untapped windings, with or without a core, in the optimum geometry. There also remains, a need to identify a Euclidian or symmetric form of hybrid transformer with windings superimposed about a single point space.